1. Field of the Invention
The present invention relates to a method for controlling particle contamination on a sample surface, and more particularly, to a method for venting a gas into a closed space without generating particles or lifting particles off surfaces therein during sample transfer from a vacuum to atmospheric environment.
2. Background of the Related Art
Charged particle beam imaging, such as electron beam imaging inspection (EBI) of a sample is typically performed in a vacuum environment. For example, the environmental pressure of the sample during imaging is equal to or lower than 1.0×10−5 Torr. For convenience of description, this definition of vacuum pressure will hold hereinafter in the specification. When the imaging step is complete, the sample needs to be transferred back to the atmospheric environment. Transition from a vacuum to atmospheric environment is typically carried out by a two step process where the sample is first disposed in a vacuumed closed space, and then an inert gas such as nitrogen is slowly vented into the closed space till the pressure inside the closed space reaches a desired level (e.g. close to the atmospheric pressure). Next, the closed space is opened, allowing the sample to be moved out and into the atmospheric environment.
As illustrated in FIG. 1, a charged particle beam imaging system 10 may include a load port 12, a sample feed chamber 14, an image forming apparatus 16 (in FIG. 2), an imaging chamber 18 and a conditioning chamber 20. The feed chamber 14 is used for pre-imaging and post-imaging storage of the sample. The load port 12 is connected with the feed chamber 14. The interested sample is fed into the feed chamber 14 through the load port 12. When imaging process is complete, the imaged sample is transferred out from the feed chamber 14 back to an external storage through the load port 12. The interior of feed chamber 14 and the load port 12 is at atmospheric pressure.
Referring to FIG. 2, the image forming apparatus 16 is mounted to the imaging chamber 18 wherein the sample is secured during imaging. The image forming apparatus 16 images the sample and form a grey level image of the sample. The interior of the imaging chamber 18 is typically at a vacuum pressure. The conditioning chamber 20, which includes a load lock structure 22 connects to the imaging chamber 18. The conditioning chamber 20 is used for preparing the sample a proper environment for the subsequent operation. For example, when the sample is to be imaged, it is first placed in the conditioning chamber 20. Then, the conditioning chamber 20 is evacuated. When the internal pressure of the conditioning chamber 20 reaches a vacuum pressure, the sample is transferred to the imaging chamber 18 for imaging. When imaging is completed, the imaged sample is then transferred back into the conditioning chamber 20, and then the internal pressure of the conditioning chamber 20 is raised. When the internal pressure of the conditioning chamber 20 substantially reaches the atmospheric pressure, the sample is transferred out and forward to the next stage of process.
Referring to FIG. 2, the load lock structure 22 includes a movable bottom 221 and a movable upper portion 222. The bottom 221 and the upper portion 222 can be moved to tight contact against each other thereby forming a closed space 223. A plurality of elastic spacers 224 are arranged on the edges of the bottom 221 and the upper portion 222 to ensure sealed contacting edges and air-tightness of the formed closed space 223. Furthermore, as shown in FIG. 2, a stage 24 is set in the imaging chamber 18, and a vacuum arm 26 is set between the stage 24 and the load lock structure 22. The vacuum arm 26 is for transferring the sample between the stage 24 and the load luck structure 22. The stage 24 is for supporting the sample thereon during charged particle beam imaging.
In practice, when a sample 28 is to be transferred from the feed chamber 14 to the imaging chamber 18 for imaging, the sample 28 is first placed on the bottom 221, and the upper portion 222 is moved to cover the bottom 221. As a result, the sample 28 is enclosed in the formed closed space 223. A pumping process is then performed to evacuate the formed closed space 223. When a desired vacuum level has been reached, the bottom 221 is lowered to open the closed space 223 and the sample 28 is transferred out to the stage 24 by the vacuum arm 26, so that an atmosphere-to-vacuum transition process is completed.
When the imaging step is complete, the sample 28 needs to be transferred back to the atmospheric environment. The transition from a vacuum to atmospheric environment, a vacuum-to-atmosphere transition process, is typically carried out by following steps. The sample 28 is first transferred from the stage 24 by the vacuum arm 26 and placed on the bottom 221 which is then lifted to contact with the upper portion 222. As a result, the sample 28 is again enclosed in the formed vacuumed closed space 223. Then, an inert gas such as nitrogen is slowly vented into the closed space 223 till the pressure inside the closed space 223 reaches a desired level (e.g. close to the atmospheric pressure). When the gas venting process is complete, the upper portion 222 is lifted to uncover and thus open the closed space 223 to allow the sample 28 to be moved out and into the atmospheric environment (i.e. to the feed chamber 14). The vacuum-to-atmosphere transition process is thus completed. When the next sample 28 is to be transferred to the imaging chamber 18 for imaging, the atmosphere-to-vacuum transition process is performed again inside the conditioning chamber 20.
During the vacuum-to-atmosphere transition process, the major problem is a frequent under-controlled particle contamination on the sample 28 surface. The inert gas is more too often easily supplied at an overly fast flow rate. The strong gas stream deforms and/or shifts the sample 28, producing tiny particles. In addition, the gas stream may also lift particles present in the closed space 223, for example stowaways carried in by the sample 28 from outside of the closed space 223.
On the other hand, the friction and deformation that occurs to the surface of the elastic spacers 224 is also suspicious for producing undesirable particles. For example, the surface of the elastic spacers 224 may wear away due to friction against the edges of the upper portion 222 or the bottom 221. Also, deformation of the bottom 221, upper portion 222, and elastic spacers 224 may produce tiny fragments. As a result, lots of fragments/particles are released from the surface of each of these elements, which then become a potential contaminant to the sample 28 surface.
Currently, to overcome this problem, the gas flow rate is controlled by using a valve with an adjustable opening diameter. FIG. 3 is a diagram illustrating operation of a gas venting process in accordance with the prior art. The valve opening is first kept small, as illustrated in stage (I) of FIG. 3, allowing a weak stream of gas flowing into the closed space. This allowed stream is so weak that it barely lifts any particles. After a certain period of time, the pressure inside the closed space comes to a saturation pressure, at which the incoming gas no longer causes disturbance and thus convection to the gas inside the closed space. The gas flow rate is then raised to a higher value, as illustrated in stage (II) of FIG. 3, to speed up the overall gas venting process. The gas venting process ends when the pressure inside the closed space comes to a desired final pressure. The desired final pressure is typically selected to be close or equal to the atmospheric pressure. It is noted that the saturation pressure is generally smaller than the desired final pressure, and can be determined by experiments with a pressure gauge.
The foregoing two-stage approach has a drawback of irreproducible results. As the opening of the valve is controlled by a mechanical parts assembly, even with identical valve setting parameters, the opening diameter can change due to, for example, attrition at the surface of involving parts of the valve. Such change in the opening size, even subtle, leads to a change in the gas flow rate which is large enough to spoil the convection characteristics inside the closed space previously observed acceptable, resulting in severe change in the amount of particles that end up being lifted. In other words, the previously selected valve settings no longer apply in the next operation from the view point of particle contamination control. As a result, the valve settings have to be determined again, for example, through experiments.
Accordingly, the conventional gas venting method has a trade-off of either being time-costly or ineffective. As the size of the semiconductor pattern shrinks significantly with the advancing fabrication technology, particle contamination on the surface of a pattern or processed feature has become more and more unacceptable. Therefore, it is desirable to provide a repeatable gas venting method.